Process for the manufacture of ceramic objects



Dec. 20, 1960 o K 2,964,822

PROCESS FOR THE MANUFACTURE OF CERAMIC OBJECTS Filed Dec. 27, 1954 FIG.2

INVENTOR DAVID E. TOMKINS' ATTORNEYS United States Patent() PROCESS FORTHE MANUFACTURE OF CERAMIC OBJECTS David E. Tomkins, New Castle, Pa.,assignor to Shenango China, Inc., New Castle, Pa., a corporation ofPennsylvania Filed Dec. 27, 1954, Ser. No. 477,634

8 Claims. (Cl. 25156) This invention relates to the manufacture ofceramic objects made wholly or partly from clay or other finegrainedsubstances.

The several processes for the forming of ceramic shapes or objects maybe classified according to the nature or state of the clay orfine-grained substance which is used. Accordingto such a classification,there are three basic types of processes: 1) slip processes; (2) plasticprocesses; and (3) dry-press processes. The dry-press processes areforeign to the subject matter of the present invention and not furtherdiscussed herein.

In the slip processes a mixture of water and clay or clays such askaolin and/or ball clay, or clay-like substances such as very finelyground deflocculated alumina, with or without additional ingredientssuch as feldspar or flint are brought to a fluid or semi-fluid statecharacterized in the art by the name slip, the slip being thenintroduced into a porous mold, which, by withdrawing water from theslip, produces green ware of the desired shape.

'Such slip processes include the method of slush casting (drain casting)used in production of hollow shapes which cannot be turned, such asovoid bowlsand jars. Slip processes also include the methods known inthe. art as gravity solid casting and pressure solid casting. An exampleof gravity solid casting is found in Wadman US. Patent No. 2,288,661,issued July 7, 1942. Examples of pressure solid casting are found inEckstein US.

Patent 1,163,328, issued December 7,. 1915; Montgomery -U.S. Patent1,864,365, issued June 21, 1932; Bower US. Patent 2,273,016, issuedFebruary 17, 1942, and

Blackburn US. Patent 1 2,669,762, issued February 1 Pressure solidcasting is distinguished from gravity ,solid casting by accelena tedremoval of water into the waterabsorptive mold. walls. Pressure feedingof the .slip results insuch hastening of water absorption, as set .forthexplicitly in Montgomerys' patent specification.

The. Bower disclosure relates to .a methodof feeding slip to the mold ata rate which varies within each feeding cycle. 'The Blackburn patentisdirected to. a quick ,mold release technique applicable bothtoconventional may be aided or accelerated by pressure feeding of theslip. According to Montgomery, pressure-fed slip V is reduced to aleather-hard state ready for removal from the mold within severalminutes. According to Blackburn, irrespective of the period of timerequired to form the solid leather-hard object within the mold (whetheri by gravity or pressure casting), additional time for natural 2,964,822v Patented Dec. 20, 1960 shrinkage must be allowed in order to separatethe shaped article from the mold without damage in the absence of thequick release technique to which his patent is directed. In thepraoticeof all the above patents, the green formed article remainssubject to typical substantial shrinkage during drying prior to firing,such shrinkage of green were or shapes being a well-known andcharacteristic phenomenon in the art. I

, In the solid slip casting processes of the prior art, referred toabove, water of suspension is filtered from the injected slip by theporous mold walls. As a layer of matter from which the water ofsuspension has been removed starts to build up, the water of suspensionof the newly incoming slip'must be forced and absorbed therethrough inorder to continue the conventional dewatering operation. The plasticagglomeration of particles within the mold builds backwardly toward theslip feed passage or passages until the mold cavity is entirely filled,such procedure requiring several minutes in the case of pressure castingand considerably more time in gravity casting. The plastic massapparently comprises a continuous water phase which surrounds eachparticle with relatively stiff plastic masses by extrusion.

a water film and through which, at intermediate stages, water ofsuspension from freshly arrived slip may con tinue to flow.

The plastic processes of the prior art include a number of types ofoperations on a mixture of water and clay or clay-like materials with orwithout additional ingredients which are brought into -a condition knownin the art as the plastic state. The plastic mass may then be jiggered,hand-molded or pressed between water-absorbent molds. Shapes may besuccessfully formed from Injection molding has been used, a finelyground non-plastic (and non-liquid), mixed with about fifteen percent ofthermosetting and thermoplastic resin, being forced from a heatedchamber into a cooled mold, the chilled piece being then heat-treated toremove the organic matter.

Considerable work has been done in connection with those plasticprocesses which employ porous molds or molds having porous facesdesigned. to absorb water from the plastic mass while confining thesolid components of the mass within the mold cavity. An early example isfound in Westman Patent 1,864,365. Another example of a type of diestructure which has recently been commercially employed is of thegeneral type illustrated in Figure 7 of Jordan Patent 2,638,654. Certaindetailed reference to these patents will be madein order that thesignificance of the present invention maybe most clearly understood. p

In Westmans disclosure, it is to be noted that the dies orpistonscomprising the fplungers 4 are forced together to the full extentnecessary to remove water .from the plastic mass. Thus, the thickness ofthe formed mass is determined by at least three factors: (1) volume ofplastic mass prior to forming; (2) water content of plastic mass priorto forming; (3) size or sizes and shapes of solid particles in plasticmass.- These three factors,

a and possibly other factors, are variable to a degree which in Westman,certain portions of the-pressed mass are more completely dewatered thanothers at the conclusion of the pressing operation. -Also,- thedegree-of .deewatering .varies fromoperation to operation. D watering isaccompanied by volume shrinkage and uneven watering and uneven pressureof forming cause warpage, strain and cracks which may appear before orafter firing. The die structure shownin Figure 7 of the Jordan patentillustrates a water-absorbent die structure in which compressing travelof the dies is positively limited. LWhile it is feasible forformingsmaller pieces of ware it ;has the;disadvantages outlined above.

'111e typeof operation illustrated in Iordans Figure 7 is dissimilar toWestman in significant respects other than theprovision of travel stop.Excess plastic mass isre- ,liev ed in gutters or channels shown indiamond-shaped cross section in Jordans Figure 7. This holds the range.of de-watering variation throughout the mass from operation tooperation within reasonable limits while allowing close thicknesscontrol. (Westman, if providedwith a .travel stop, would vary morewidely in degree of dewatering from piece to piece because of theabsence of such relief.) At the same time, the relief provided in theJordan operation limits the molding pressure which may be achieved andoccasions some unevenness of pressures insofar as portions of the cavity(the rim in Jordans Figure 7) are relieved from pressure to a greaterextent than other portions (center portion of cavity in Jordans Figure7). The type of operation illustrated in Iordans Figure 7-is alsodissimilar to- Westman in that a limited amount of moisture isdeliberately blown back to, thedie faces to effect ware release. It isto be noted that the Westman patent (see particularly page 2, column 1,lines 19-24)- teaches that water in contact with the mass will workagainst-the de watering sought to be achieved, particularly at highermolding pressures. However, the blow-back of a relatively small amountof moisture for ware releasing purposes as in Jordan has been found tobe not too objectionable. Blow-back of a small amount of moisture forware-releasing purposes is also shown by Blackburn in connection withsolid casting.

The present invention relates to a slip process and embodies thediscovery that, entirely contrary to the teachings of Westman in regardto moisture contact, the soil mechanics of slips are such that surfacetension orparticlc-water interface effects may be overcome. andshrinkage water may actually be expressed from between the clayparticles or other fine particles comprised in the slip within a porousmold even when employing as the compressing medium a liquid orsuspension (which may most simply be additional slip succeeding thatsupplied through an input line to the mold cavity) provided that thecompressive medium is applied to the slip within the porous mold atinvestment pressures greatly exceeding those employed inthe most closelyrelated prior art-dealing with slip-processes. Such removal of shrinkagewater is supplemental to removal of water of suspension. Although wateris the essential compressing medium, the slip par- -ticles within themold cavity actually impinge together under the high imposedfluid'pressure, and upon release of pressure following formation of thearticle and previ- -ous to its removal'from the mold there is nosubstantial -reversal of the de-watering action despite the direct con-'tact of the de-watered shrunk article with the fluid -which imposed thepressure. Thus, in a sense, the overcoming of surface tension orparticle-water interface effects so as to eliminate shrinkage water inaddition to -water of suspension may be said to be irreversible or, moreaccurately, non-reversing in the practice of the in- .vention.

Theinvention also embodies the discovery thatsuch on-reversibility, asreferred to just above, obtains even when'substantially the entiresurface area of an article .rwhich is deprived of substantially allshrinkage water as :well as water of suspension is subjected to directwater contact, as for releasing purposes.

As used herein the term shrinkage water is used to designate that waterthe-removal of which is accompanied ;-byeshginkage of theclay.bodyz-fromcwhichzsllch waterzis removed (which body has previouslybeen rendered incapable of fluid flow by removal of water ofsuspension).

The invention embodies the further discovery that, at elevatedpressures, de-Watering may continue to occur under a fluid-imposedpressure despite the progressively closer packingof the clay orclay-like particles. It is not clear whether expressing of the shrinkagewater is delayed until the mold is almost filled with a mass from whichwater of suspension has been removed, at which time such expressingtakes place more or less simultaneously throughout the mass, or whetherde-watering continues through the intersticial spaces remaining betweenthe closely packed particles of outer'laminae after removal of all ornearly all shrinkage water in such laminae. In any event, the residuumof matter remaining within the mold constitutes a substantiallypreshrunk article, thus avoiding all the problems and disadvantages ofthe heretofore characteristic shrinkage of newly-formed green ceramicshapes and objects, while at the same time close thickness control isachieved. Pro-shrunk articles formed .according to the present inventioncontain very little, if

any, shrinkage water, the process removing most of the .film of watersurrounding each particle by overcoming the surface tensions involved sothat substantially all remaining water fills voids between touchingparticles.

Not only is drying shrinkage eliminated, but drying time is also greatlyreduced. Accordingly, the invention avoids all the problems ofnon-uniformity of results or of warping, uneven drying, unmatchedshrinkage of thick and thin sections, and irregular shapes, whichproblems have heretofore plagued the art.

a of varying thickness because throughout the de-watering ,pressureinvestment of the slip.

operation there is no significant variation in pressure between thickand thin portions of the section. Also, the constant supply of incomingslip under pressure completely eliminates all trace of air bubbles inthe molded articleit de-airs in addition to de-watering.

The. invention is also superior to the several plastic molding processesof the prior art because the invention,

unlike such plastic molding processes, involves a high degree ofpreferred orientation of the clay particles of jthe molded piece forgreater strength, smoothness, and

firing shrinkage uniformity and control.

The actual molding operation to which the invention relates compares inspeed with the pressing operation in plastic processes such as those towhich Westman and Jordan relate. Furthermore, this comparison of speedof-operation is entirely apart from the saving in time and overheadeffected by the invention by its elimination of any necessity ofblunging and otherwise preparing a plastic mass (Westman blunges foreight hours in his example). Ware pieces may be formed according to thepresent invention within several seconds following initiation of Incomparison with prior slip casting methods, formation of the piece inthe present invention is much more rapid (Montgomery required severalminutes) The invention is applicable to the production of those .shapesusually produced by jiggen'ng and solid casting. It eliminates thenecessity of filter presses, pug mills, elaborate clay-deliveryequipment and large mold ining equipment maybesimplified. Smallproduction runs of miscellaneous sizes and shapes may be made economi--;cally. Scrap return is virtually eliminated.

. The. inventionis adaptable toautomaticland continuous production andto the production of pieces having walls of varying thickness or anyarbitrary shape. It produces a leather-hard piece with substantially nodrying shrinkage. The process removes most of the film of water'surrounding each clay particle known as shrinkage water, substantiallyall the remaining water filling voids between touching clay particles.As explained below, the invention also makes possible several novelmeans of decoratmg mass produced ware.

It is to be understood that the invention would be applicable to theforming of ceramic pieces by use of suitable vehicles or suspendingagents other than water, and accordingly the word water throughout thespecification and claims is to be regarded as embracing both wateritself and other equivalent vehicles or suspending agents.

The objects and advantages of the invention as described above willbecome more apparent from the following specification of the invention.The means which have actually been employed to practice the inventionare illustrated schematically in the accompanying drawings. Whileadditional work may be expected to yield refinements in the invention,the herein disclosed specific em bodiment describes the most desirableform of the invention of which I am presently aware and will enablethose skilled in the art to successfully duplicate my practice of theinvention.

Figurel illustrates an embodiment of the invention which has beenexperimentally tested. Figures 2 and 3 are highly ,schematicillustrations of the sequence of operations in the practice of theinvention.

In the apparatus illustrated in Figure 1, a porous die or mold having aparting line 11 is provided. The mold 10 may be a plaster mold of thetype heretofore used in plastic pressing operations being suitablysupported by metal cladding and having inset fiberglass tubing 18communicating with the outside of the mold. The permeable walls of thefiberglass tubing pass water absorbed by the mold 10, allowing suchwater to drain from the mold assembly. An air pressure supply means (notshown) may also communicate with the tubing 18. A

supply line 12, preferably a metal line, communicates.

with the mold interior and leads from the slip valve 13. A slip supplyline 14 supplying slip at suificient pressure to cause flow communicateswith the valve 13. A tank 15 is provided having a central flexiblediaphragm 16, the lower portion of the tank 15 communicating with thevalve 13 through a line 17. The valve 13 is actuated by a solenoid 19which, when energized, shifts the valve 13 to the illustrated position.The upper portion of the tank 15 communicates through a line 23 with anoil valve 24 to which are ported a line 25 leading from a source of oilpressure and a dump line 26 leading to an oil reservoir. The oilpressure may be supplied by conventional pump means having a suitableadjustable relief valve enabling any desired pressure over a wide rangeto be selected and maintained, as will be obvious to those familiar withhydraulic control systems. An adjustable flow control valve 22 isinserted in the line 23 to which may also be attached a suitablepressure gauge 27. The valve 24 is actuated by a solenoid 28 which, whenenergized, shifts the valve 24 to the illustrated position.

A hand-operated press control valve 31 is provided. Ported to this valveare the lines 32 and 33 leading to opposite ends of a hydraulic cylinder20, which is adapted to actuate the upper die of the mold 10 and to holdthe die in open or closed position. Also ported to the control valve 31are line 35 leading from a source of hydraulic fluid pressure and line36 leading to a hydraulic fluid reservoir. The oil supplied to the tank15 and the hydraulic fluid supplied to the cylinder may or may not beidentical; actual experimental practice of the invention happens to havebeen performed with different liquids and independent pressure supplysystems.

A pressure switch 37 is ported to the line 32 and is adapted to beclosed when pressure in this line rises above 6 a predetermined'valueand opened when pressure falls below this value. The solenoids 19 and 28are in series with this pressure switch.

When the handle 31 is in the position illustrated, hydraulic fluid isadmitted to the hydraulic cylinder through the line 32, the top die ofthe mold 11 being lowered into closed position as shown in Figure 1.When the top die can no longer move, pressure in the line 32 increasesand causes the pressure switch 37 to energize the solenoids 28 and 19.The valves 13 and 24 thereupon assume the positions illustrated inFigure 1. Oil at a molding pressure of 600 p.s.i. or over is admitted tothe top half of the tank 15, the rate of flow being controlled by thevalve 22. Slip in the bottom half of the tank 15 is forced through theline 12 and into the mold at the same pressure. Water filtered from theslip within the mold is forced through the mold and allowed to drainaway through the tubes 18. Slipwithin the mold 10 is transformed into apro-shrunk leather-hard shaped mass from which most or all of theshrinkage water has been removed, the clay or clay-like particles ofthis mass being oriented along their directions of flow into finalposition or generally along directions parallel to the interior moldsurfaces. Apparently such particle orientation, by reducing the volumeof voids between particles, minimizes the volume of Water remainingwithin the mass follow-. ing removal of shrinkage water. Minimizing ofsuch volume further reduces the residual moisture within the formedarticle, thus further shortening drying time. Relatively little waterescapes from the slip within the line 12 because of the confining effectof the metal walls of this line.

After a short time interval, not usually exceeding several seconds,compressed air may be momentarily admitted into the lines 18 to blowmoisture back to the die faces to enable ready release of the formedpiece of ware, and the valve 31 may be reversed, causing the upper molddie to be raised. With the corresponding pressure drop in the line 32,the solenoids 19 and '28 are de-energized, causing the valves 13 and 24to reverse and allowing a fresh supply of slip from the line 14 to beadmitted in the tank 15. The formed piece is removed from the mold 10,and the'apparatus is ready for a succeeding molding operation.

In oneexperirnent, six-inch diameter cereal bowls were produced asfollows. A slip was provided consistingof thefollowing ingredients, allof solid ingredients havmg been passed through a 150 mesh screen:

Percent by weight This slip was prepared in the conventional manner andwas supplied from the slip feed line at a pressure of 70 p.s.i. Duringmolding, oil pressure of 750 p.s.i. was applied to the oil-slip tank.Slip was fed into the mold under pressure for 15 seconds. At the end ofthis time, compressed air at p.s.i. was ported to the fiberglass tubing,and the mold was opened and a piece of ware, of leather-hardconsistency, was removed from the mold. The pieces were thereupon driedand were fired for 60 hours at 2240 F. in a sanded bung. No apparentwarping occurredduring firing. The surfaces of the fired pieces wereexamined and found to reproduce with high definition extremely small andshallow flawsor steps in the interior surface of, the experimental moldrThe plaster mold used in this experiment was made as follows:

Five parts Ceramical and two parts water were mixed approximately 30minutes with a power mixer until the mixture became thick and creamy.The mixture was then poured into a mold pattern in which was suitablyposit ioned the fiberglass tubing, this tubing being arranged to layabout 4 inch below the working surface of the mold. The mold was allowedto set for ten minutes after which a small amount of air was blownthrough the fiberglass tubes to purge the mold.

The schematic illustrations in Figures 2 and 3 are-intended toillustrate the broader aspects of the invention. In Figure 2, a mold 100is held closed by a set of clamps 101. Clay slip is injected into themold at about 750 p.s.i. through the line 102, water of suspension andshrinkage water being removed from the injected matter and particleorientation further minimizing the moisture residue asabove described.Following this step, the clamps are released, the mold is opened, andthe molded piece 105 is removed from the mold as shown in Figure 3. Ingeneral, with the slip compositions I have worked with, moldingpressures may vary upwardly from about 600 p.s.i., with the presentlymost feasible pressures, from the standpoint of original cost andmaintenance cost of presently available components, on the one hand, andresults and production time on the other, having a practical top limitinthe neighborhood of 1,000 p.s.i.

The pumping means employing the diaphragm-tank 15 illustrated in Figure1 was provided to overcome the extreme problems'of'erosion and wearwhich arise when bearing. surfaces are subjected to clay slip,particularly when such exposure, occurs under high pressure. Otherpressure systems adapted to handle highly abrasive fluids or semi-fluidsmay be employed, although the illustrated system appears to be welladapted to the intermittent type of operation employed in theillustrated embodiment.

The advantages of the invention from the standpoint of ware decorationshould be mentioned. The inner surfaces of the-open porous mold may bespattered before closing with a colored slipproducing a spatter effectin the final product. The same effect may be achieved by injectedcolored slip into the closed mold prior to introduction of the main bodyof slip, such colored slip being introduced either under very highdirect pressure or with compressed air. Marbleized designs may beobtained through thealternate injection of slips of different colors.

The invention is broadly applicable to' the production of all types ofceramic shapes, including bricks, tiles, sanitary ware and other ceramicproducts in addition to pottery ware. The several variations thereofdescribed above will demonstrate that the scope of the invention extendsbeyond the details of the specific embodiment which I have practiced andherein disclosed and will make it clear that the scope of the inventionis not limited by the precise details of such specific embodiment. WhatI claim as my invention is defined in the following claims.

What is claimed is:

1;.A process for making green ceramic objects which comprisestransforming a ceramic slip composition to a non-fluid state byinjecting said composition into a closed sectional water-permeable moldunder a pressure sufficient to express both water of suspension andsubstantially all shrinkage water through the shaping faces of saidmold, whereby preshrunk green ware of the shape of said mold is formedwithin said mold, thereupon releasing said pressure and opening saidmold to remove therefrom, still in preshrunk condition, the formed greenware.

2. A process for making green ceramic objects which comprisestransforming a ceramic slip composition to a non-fluid state byinjecting said composition into a closed sectional permeable mold, thefaces of which pass the waterin the slip but prevent the escape of solidingredients of the slip, under-apressure sufliicient toexpress bothwater of suspension and substantially all shrinkage water through theshaping faces of said mold, whereby preshrunk green ware of the shape ofsaid mold is formed within said mold, thereupon releasing said pressureand opening said mold to remove therefrom, still in preshrunk condition,the formed green ware.

3. The method of making green ceramic objects comprising the steps ofproviding a slip, injecting the slip under pressure into a closedsectional permeable mold, the faces of which pass the water in the slipbut capture for agglomeration the solid ingredients of the slip, and

continuing pressure injection of the slip under a pressure high enoughto express substantially all shrinkage water as well as water ofsuspension from the agglomerating solid ingredients of the slip, wherebypreshrunk green ware of the shape of said mold is formed within saidmold, thereupon releasing said pressure and opening said mold to removetherefrom, still in preshrunk condition, the formed green ware.

4. A process for making green ceramic objects comprising the steps ofproviding slip capable of fluid flow when-subjected to pressures above agiven value whether or not capable of fluid flow when subjected topressures below said given value, injecting the slip under pressureabove said given value into a closed sectional permeable mold, the facesof which pass the water in the slip but capture for agglomeration thesolid ingredients of the slip, and continuing pressure injection of theslip under a pressure sufiiciently above said given value to expresssubstantially all shrinkage water as well as water of suspension fromthe agglomerating solid ingredients of the slip, whereby preshrunk greenware of the shape of said mold is formedwithin said mold, thereuponreleasing said pressure and opening said mold to remove therefrom, stillin preshrunk condition, the formed green ware.

5. A process for making green ceramic objects which comprisestransforming a ceramic slip composition to a non-fluid state byinjecting said composition into a closed sectional water-permeable moldunder apressure suflicient to express both water of suspension andsubstantially all shrinkage water through the shaping faces of saidmold, whereby preshrunk green ware of the shape of said mold is formedwithin said mold, thereupon performing the terminal operations ofreleasing said pressure and opening said mold, and blowing moisture backto the shaping faces of said mold incident to said terminal operationsto effect ready release and removal from the mold of the formed greenware, still in preshrunk condition.

6. A process for making green ceramic objects which comprisestransforming a ceramic slip composition to a non-fluid state byinjecting said composition into a closed sectional permeable mold, thefaces of which pass the water in the slip but prevent the escape ofsolid ingredients of the slip, under a pressure suflicient to expressboth water of suspension and substantially all shrinkage water throughthe shaping faces of said mold, whereby preshrunk green ware of theshape of said mold is formed Within said mold, thereupon performing theterminal operations of releasing said pressure and opening said mold,and blowing moisture back to the shaping faces of said mold incident tosaid terminal operations to effect ready release and removal from themold of the formed green ware, still in preshrunk condition.

7. The method of making green ceramic objects comprising the steps ofproviding a slip, injecting the slip under pressure into a closedsectional permeable mold, the faces of which pass the water in the slipbut capture f(l' agglomeration the solid ingredients of the slip, andcontinuing pressure injection of the slip under a pressure high enoughto express substantially all shrinkage water as well as water ofsuspension from the agglomerating solid ingredients of the slip, wherebypreshrunk green ware of the shape of said mold is formed within saidmold, thereupon performing the terminal operations of re-Ieasing-saidpressurc and openingsaid mold, and blowing moisture back tothe shaping faces of said mold incident to said terminal operations toefiect ready release and removal from the mold of the formed green ware,still in preshrunk condition.

8. A process for making green ceramic objects comprising the steps ofproviding slip capable of fluid flow when subjected to pressures above agiven value whether or not capable of fiuid flow when subjected topressures below said given value, injecting the slip under pressureabove said given value into a closed sectional permeable mold, the facesof which pass the water in the slip but capture for agglomeration thesolid ingredients of the slip, and continuing pressure injection of theslip under a pressure sufliciently above said given value to expresssubstantially all shrinkage water as well as water of suspension fromthe agglomerating solid ingredients of the slip, whereby preshrunk greenware of the shape of said mold is formed within said mold, thereuponper- References Cited in the file of this patent UNITED STATES PATENTS1,340,308 Williams May 18, 1920 1,993,047 Westman Mar. 5, 1935 2,669,762Blackburn et al. Feb. 23, 1954 2,770,025 Mollers Nov. 13, 1956 OTHERREFERENCES Encyclopedia of the Ceramic Industries, volume 3, page 130,Searle, published by Ernest Benn, Ltd., London, 1930. (Copy available inDivision 15.)

